The present invention relates to a vacuum power interrupter and, more particularly, a vacuum power interrupter manufactured by using the most suitable vacuum hermetical brazing material as the material of each component.
Generally a, vacuum power interrupter such as vacuum circuit breaker is constructed by fixing an upper end plate and a lower end plate to both axial ends of a cylindrical insulating envelope respectively, mounting a bellows on the lower end plate, supporting a movable contact rod having a movable electrical contact at the top thereof to the bellows, and incorporating a stationary contact rod having a stationary electrical contact at the bottom thereof with the upper end plate.
In manufacturing the above mentioned vacuum power interrupter between, each component namely between the cylindrical insulating envelope and the upper end plate, between the bellows and the lower end plate, between the bellows and the movable contact rod, between the upper end plate and the stationary contact rod, between the stationary contact rod and the stationary electrical contact, between the movable contact rod and the movable electrical contact there is a suitable vacuum hermetical brazing material.
Various kinds of vacuum hermetical brazing materials are used according to the material of the components of the vacuum power interrupter. The results of applying various standard vacuum brazing materials (braze temperature range between 600.degree. C and about 1000.degree. C) to materials to be brazed are shown in Table 1 where mark "o" means proper, mark "x" means improper, m.p. is the melting point of the vacuum brazing material and f.p. is the flow point thereof.
It is well known from Table 1 that vacuum hermetical brazing materials of low melting point, such as indicated by No. 1 through No. 4 in Table 1, cannot be used in an alloy composed of iron and chromium (which will be called "Fe-Cr alloy" hereinafter) of high braze temperature range. And it is also well known from Table 1 that vacuum hermetical brazing materials of high melting point, such as indicated by No. 5 through No. 9 in Table 1, cannot be used in an alloy composed of silver (which will be called "Ag alloy" hereinafter) of low braze temperature range. It is therefore concluded that the braze temperature range strongly depends on the material of the bellows made of Fe-Cr alloy and the material of the electrical contacts made of an alloy composed of copper (which will be called "Cu alloy" hereinafter) or Ag alloy.
Table 1 __________________________________________________________________________ vacuum brazing materials to be brazed materials m.p. f.p. Fe-Co Fe-Ni Fe-Cr F3-Ni-Co Cu Ag No. (Wt%) (.degree. C) (.degree. C) Cu Ni Fe alloy alloy alloy alloy alloy alloy __________________________________________________________________________ 1 61Ag-24Cu-15In 630 705 o o o o o x o o o 2 60Ag-27Cu-13In 605 710 o o o o o x o o o 3 72Ag-28Cu 780 780 o o o o o x o o o 4 20Ag-60Au-20Cu 835 845 o o o o o x o o o 5 80Au-20Cu 890 890 o o o o o o o o x 6 53Cu-38Mn-9Ni 880 910 o o o o o o o o x 7 82Au-18Ni 950 950 o o o o o o o o x 8 100Ag 960 960 o o o o o o o o x 9 85Ag-15Mn 960 970 o o o o o o o o x __________________________________________________________________________
With respect to the vacuum braze temperature, generally heat from heat source is transmitted by radiation to a heated portion. Therefore, it is difficult to heat the heated portion in a short time. Moreover, it is necessary to add 50.degree. C to the temperature of the flow point as the actual vacuum braze temperature due to the requirement for uniform heating of the heated portion.
The above examination therefore tells us, in the case of simultaneously brazing each component of a vacuum power interrupter, that the vacuum braze temperature must be set based on the lowest temperature of the material of the bellows, assuming that the material of the contact rods are made of Cu alloy. Accordingly, in this case the suitable vacuum hermetical brazing material can be that indicated by No. 5 through No. 9 in Table 1.
This establishes that, a vacuum power interrupter by simultaneously brazing each component, the material of the contact rod must be a Cu alloy having a wide braze temperature range and the vacuum braze temperature must to be set in accordance with the lowest temperature of the material of bellows made of Fe-Cr alloy of higher braze temperature than any other components. Moreover, in execution of the vacuum hermetical brazing, it is desirable to keep it completely hermetical for high reliability of the vacuum power interrupter.
An examination of the influences of the vacuum hermetical brazing material including Ag (which will be called "Ag brazing material" hereinafter) on Fe-Ni alloy shows that, if there is any melted Ag brazing material on the surface of the Fe-Ni alloy, percolation of the Ag brazing material into the grain boundary of the base material proceeds to a high degree and if there is any tension (external force or internal force, such as thermal stress which occurs when brazing metals of different coefficients of thermal expansion) applied to the base material, percolation of the Ag, brazing material into the grain boundary of the base material is apt to occur and such phenomena tends to generate cracks in the base material.
In order to investigate these causes, after brazing the base material without a plating or metal clad thereon, the presence of cracks in the base material based on percoration of the Ag brazing material into the grain boundary of the base material has been examined. The braze temperature is suitably set in accordance with the brazing material and the base material, and the base material is subjected to a tension stress between 1 Kg/mm and 12 Kg/mm. The result of this examination is shown in Table 2 where mark "o" indicating an unchanged condition, mark "x" indicates the presence of cracks and mark ".DELTA." indicates negligible presence of cracks. Brazing material including Cu will be called "Cu brazing material" and brazing material including Au will be called "Au brazing material" hereinafter and neither the Cu brazing material nor the Au brazing material includes Ag.
Table 2 ______________________________________ base material Fe-Ni-Co Fe-Ni Fe-Cr brazing material alloy Fe alloy alloy Cu ______________________________________ Ag brazing material x x x .DELTA. o Cu brazing material o o o o o Au brazing material o o o o o ______________________________________
It is seen from Table 2 that cracks occur only when using Ag brazing material. If the value of the tension is changed within the above range the, same results have been obtained except that the time of the cracks changes respectively. According to circumstances, cracks which have occurred on the base material penetrate into the base material and consequently the base material is often cracked.
Next, in the case of the use of Ag brazing material as the base material with an Ni plating thereon the, occurrence of cracks on the base material under the same conditions as the above, the results are shown in Table 3 where mark "o" indicates an unchanged condition and mark "x" indicates the presence of cracks.
Table 3 ______________________________________ brazing thickness of Fe-Ni-Co Fe-Ni Fe-Cr condition Ni plating alloy Fe alloy alloy ______________________________________ 3 .mu. x x x o in vacuum 5 .mu. x x x o 8 .mu. x x x o in hydrogen 3 .mu. o o o o ______________________________________
It is concluded that in vacuum hermetical brazing, it is inevitable to generate cracks on a base material made of Fe-Ni-Co alloy and Fe-Ni alloy in spite of an Ni plating thereon, whereas in the case of execution of hydrogen brazing, there are no cracks at all.
The cross sectional region of the brazed portion of the base material in the case of vacuum hermetical brazing and hydrogen brazing has been obserbed. In the case of the vacuum brazing material the, Ni plating layer is extricated from the surface of the base material by erosion of the brazing material and consequently the Ag brazing material percolates into the interstice formed by the erosion and directly contacts the base material. For this reason percoration into the grain bundary of the base material comes cracks. The same results are obtained, in the case of making the Ni plating layer thicker up to a value for example over 10 .mu.. This considerable thickness of the Ni plating layer on the base material results in damage to the plating tightness, increases the difference of thermal expansion, and increases the production time and costs of production. Therefore, the application of the Ni plating to base material in vacuum hermetical brazing is not favorable.
On the contrary, hydrogen brazing is free from the faults mentioned above because the Ni plating layer prevents the Ag brazing material from percolating into the base material. The difference between vacuum brazing and hydrogen brazing with respect to the influence of the Ag brazing material on the base materials mainly attributed to respective temperature conditions. Namely in hydrogen brazing, the heated portion is heated rapidly by conduction and radiation. For this reason, the heating time is short, for example within one minute and the blazing temperature can be set at the flow point or a little higher. Additionally due to a good cooling effect, the melting time of the brazing material is accordingly shortened, and substantial diffusion and percolation of the brazing material does not occur.
However, in vacuum brazing as the heated portion is heated only by radiation, it is difficult to heat this portion in a short time. Therefore, the brazing temperature in vacuum brazing is remarkably higher than that in hydrogen brazing and the cooling effect is considerably lowered. Therefore, the melting time of the brazing material is lengthened, for example, about half an hour, and diffusion and erosion of the brazing material into the base material is increased, and consequently cracks are apt to occur.
Further examination of the vacuum brazing material including Ag reveals the following; Namely examination on the causes of cracks in the base material in applying Ag brazing material to a base material made of Fe-Ni-Co alloy, Fe and Fe-Ni alloy will be made. In the case of melting an Ag brazing material on the surface of a base material, the Ag brazing material percolates into fine cracks or coarse portions of the base material and percolates into the wedge shaped grain boundary of the base material produced by such percolation and diffusion of the brazing material, and thereby cracks occur in the base material.
The above percolation and diffusion of the brazing material are promoted by the affinity and sensitivity between the base material and the Ag brazing material. If there is any tension applied to the base material, such percolation and diffusion as above mentioned will be further promoted.
On the contrary, Cu brazing material and Au brazing material which do not include Ag, disperse finely and uniformly percolate into the crystal grain boundary of the base material. A favorable diffusion layer for brazing is thus formed. Selective percolation into the grain boundary in the Ag brazing material is not observed. Even if a tension is applied to the base material, cracks do not occur. Cu brazing material and Au brazing material which do not include Ag, are applicable to a brazing material for brazing vacuum power interrupter which must have reliability of sealing. Of course, it is unnecessary to form an Ni plating on the base material.
Though hydrogen brazing has various advantages as above mentioned, some drawbacks in brazing are as follows;
a. It is necessary to evacuate at the predetermined degree of vacuum after hydrogen brazing. Therefore an additional manufacturing process, which is unnecessary in vacuum brazing, is required.
b. In hydrogen brazing work it is necessary to prevent any dangerous events from occurring.
c. A hydrogen brazing material which includes Mn, is improper as a vacuum brazing material as a result of the reaction between hydrogen and Mn.
A detailed analysis of the above reveals the following;
a. Ag brazing material is not suitable for brazing between end plates made of Fe-Ni alloy or Fe-Ni-Co alloy and cylindrical insulating envelope made of ceramics. On the contrary Cu brazing material is suitable for brazing such portion.
b. Cu brazing material or Au brazing material is suitable for brazing between the lower end plate and the bellows, and between the lower end plate and the arc-shield member.
c. Cu brazing material or Au brazing material is suitable for brazing between the bellows made of Fe-Cr alloy and a movable contact rod made of Cu.
d. Cu brazing material, Au brazing material or Ag brazing material is satisfactory for brazing between electrical contacts made of a Cu alloy and a contact made of Cu.
However, if the components of a vacuum interrupter are to be brazed simultaneously, brazing materials having overlapping ranges of braze temperature should be used and therefore a Cu brazing material or an Au brazing material is suitable in this case.